Evolution and Adaptation

by Thomas Hunt Morgan

Of far greater value than these speculations as to the origin of sex are the experiments that appear to show that nutrition is an important factor in determining sex. Some of the earlier experiments in this direction are those of Born and of Yung. By feeding one set of tadpoles with beef, Yung found the percentage of females that developed to be greatly increased, and a similar increase was observed when the tadpoles were fed on the flesh of fish. An even greater effect was produced by using the flesh of frogs, the percentage rising to 92 females in every hundred. These results have been given a different interpretation by Pflüger and by others, and, as will be pointed out later, there is a possible source of error that may invalidate them.

Somewhat similar results have been obtained by Nussbaum for one of the rotifers. He found that if the rotifer is abundantly fed in early life, it produces female eggs, that is, larger eggs that become females; while if sparingly fed, it produces only small eggs, from which males develop. It has been claimed also in mammals, and even in man, that sex is to some extent determined by the nourishment of the individual.

Some experiments made by Mrs. Treat with caterpillars seemed to show that if the caterpillars were well nourished more female moths were produced, and if starved before pupation more males emerged. But Riley has pointed out that since the larger female caterpillars require more food they will starve sooner than the males, and, in consequence, it may appear that proportionately more male butterflies are born when the caterpillars are subjected to a starvation diet. This point of view is important in putting us on our guard against hastily supposing that food may directly determine sex. Unless the entire number of individuals present at the beginning of the experiment is taken into account, the results may be misleading, because the conditions may be more fatal to one sex than to the other.

In some of the hymenopterous insects, the bees for example, it has been discovered that the sex of the embryo is determined by the entrance, or lack of entrance, of the spermatozoon. In the honey-bee all the fertilized eggs produce females and the unfertilized eggs males. The same relation is probably true also in the case of ants and of wasps. In the saw-flies, the conditions are very remarkable. Sharp gives the following account of some of these forms:[^[35]]—“It is a rule in this family that males are very much less numerous than females, and there are some species in which no males have been discovered. This would not be of itself evidence of the occurrence of parthenogenesis, but this has been placed beyond doubt by taking females bred in confinement, obtaining unfertilized eggs from them, and rearing the larvæ produced from the eggs. This has been done by numerous observers with curious results. In many cases the parthenogenetic progeny, or a portion of it, dies without attaining full maturity. This may or may not be due to constitutional weakness, arising from the parthenogenetic state. Cameron, who has made extensive observations on this subject, thinks that the parthenogenesis does involve constitutional weakness, fewer of the parthenogenetic young reaching maturity. This, he suggests, may be compensated for—when the parthenogenetic progeny is all of the female sex—by the fact that all those that grow up are producers of eggs. In many cases the parthenogenetic young of Tenthredinidæ are of the male sex, and sometimes the abnormal progeny is of both sexes. In the case of one species—the common currant-fly, Nematus ribesii—the parthenogenetic progeny is nearly, but not quite always, entirely of the male sex; this has been ascertained again and again, and it is impossible to suggest in these cases any advantage to the species to compensate for constitutional parthenogenetic weakness. On the whole, it appears most probable that the parthenogenesis, and the special sex produced by it, whether male or female, are due to physiological conditions of which we know little, and that the species continues in spite of the parthenogenesis rather than profits by it. It is worthy of remark that one of the species in which parthenogenesis with the production of males occurs—Nematus ribesii—is perhaps the most abundant of saw-flies.”

[35]. “The Cambridge Natural History,” Vol. V, “Insects,” by David Sharp.

It has been pointed out that in a number of species of animals and plants only parthenogenetic females are present at certain times. In a sense this means a preponderance of one sex, but since the eggs are adapted only to this kind of development, it may be claimed that the conditions in such cases are somewhat different from those in which eggs that would be normally fertilized may develop in the absence of fertilization. Nevertheless, it is generally supposed that the actual state of affairs is about the same. It is usually assumed, and no doubt with much probability, that these parthenogenetic forms have evolved from a group which originally had both male and female forms. One of the most striking facts in this connection is that in the groups to which these parthenogenetic species belong there are, as a rule, other species with occasional parthenogenesis, and in some of these the males are also fewer in number than the females.

In the aphids, the parthenogenetic eggs give rise during the summer to parthenogenetic females, but in the autumn the parthenogenetic eggs give rise without fertilization both to males and to females. It appears, therefore, that we can form no general rule as to a relation between fertilization and the determination of sex. While in certain cases, as in the bees, there appears to be a direct connection between these two, in other cases, as in that of the aphids just mentioned, there is no such relation apparent.

Geddes and Thompson have advocated a view in regard to sex which at best can only serve as a sort of analogy under which the two forms of sex may be considered, rather than as a legitimate explanation of the phenomenon of sex. They rest their view on the idea that living material is continually breaking down and building up. An animal in which there is an excess of the breaking-down process is a male, and one that is more constructive is a female. Furthermore, whichever process is in the excess during development determines the sex of the individual. Thus, if conditions are very favorable, there will be more females produced; but if, on the other hand, there is an excess of the breaking-down process, males are produced. So far, the process is conceived as a purely physiological one, but to this the authors then apply the selection hypothesis, which, they suppose, acts as a sort of break or regulation of the physiological processes, or in other words as a directive agent. They state: “Yet the sexual dimorphism, in the main, and in detail, has an adaptive significance, also securing the advantages of cross-fertilization and the like, and is, therefore, to some extent the result of the continual action of natural selection, though this may, of course, check variation in one form as well as favor it in another.” Disregarding this last addition, with which Geddes and Thompson think it necessary to burden their theory, let us return to the physiological side of the hypothesis. Their idea appears to me a sort of symbolism rather than a scientific attempt to explain sex. If their view had a real value, it ought to be possible to determine the sex of the developing organism with precision by regulating the conditions of its growth, and yet we cannot do this, nor do the authors make any claim of being able to do so. The hypothesis lacks the only support that can give it scientific standing, the proof of experiment.

There have been made, from time to time, a number of attempts to show that the sex of the embryo is predetermined in the egg, and is not determined later by external circumstances. In recent years this view has come more to the front, despite the apparent experimental evidence which seemed in one or two cases to point to the opposite view. One of the most complete analyses of the question is that of Cuénot, who has attempted to show that the sex of the embryo is determined in the egg, before or at the time of fertilization. He has also examined critically the evidence that appeared to show that external conditions, acting on the embryo, may determine the sex, and has pointed out some possible sources of error that had been overlooked. The best-known case is that of the tadpole of the frog, but Cuénot shows not only that there are chances of error in this experiment as carried out, but also, by his own experiments and observations, that the facts themselves are not above suspicion. He points out that at the age at which some of the tadpoles were when the examination was made, it was not always possible to tell definitely the sex of the individual, and least of all by means of the size alone of the reproductive organs, as was supposed, in one case at least, to be sufficient. In his own experiments he did not find an excess of one sex over the other as a result of feeding.

Cuénot points out that Brocadello found that the larger eggs laid by the silkworm give rise to from 88 to 95 per cent of females, and the small eggs to from 88 to 92 per cent of males. Joseph has confirmed this for Ocneria dispar, and Cuénot himself also reached this conclusion. Korschelt found that the large eggs of Dinophilus produced females and the small ones males. Cuénot experimented with three species of flies, and found that when the maggots were well nourished the number of the individuals of the two sexes was about equal, and when poorly nourished there were a few more females in two cases, and in another about the same number of males and females.